<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .hei {
            background-color: #000;
        }

        .bai {
            background-color: #fff;
        }

        .btn {
            margin-left: 50%;
        }
    </style>
</head>

<body>
    <button class="btn">开关</button>
    <script>
        var btn = document.querySelector('.btn');
        var se = document.querySelector('body')
        var flag = 0;
        btn.onclick = function () {
            if (flag == 0) {
                se.className = 'hei';
                flag = 1;
            } else {
                se.className = 'bai';
                flag = 0;
            }
        }
    </script>
</body>

</html>